Abstract

We present simultaneous multi-color optical photometry using of the transiting exoplanet KIC 12557548 b (also known as KIC 1255 b) ULTRACAM. This reveals, for the first time, the color dependence of the transit depth. Our g' and z' transits are similar in shape to the average Kepler short-cadence profile and constitute the highest-quality extant coverage of individual transits. Our Night 1 transit depths are 0.85 ± 0.04% in z'; 1.00 ± 0.03% in g'; and 1.1 ± 0.3% in u'. We employ a residual-permutation method to assess the impact of correlated noise on the depth difference between the z' and g' bands and calculate the significance of the color dependence at $3.2\sigma $. The Night 1 depths are consistent with dust extinction as observed in the ISM, but require grain sizes comparable to the largest found in the ISM: 0.25–1 μm. This provides direct evidence in favor of this object being a disrupting low-mass rocky planet, feeding a transiting dust cloud. On the remaining four nights of observations the object was in a rare shallow-transit phase. If the grain size in the transiting dust cloud changes as the transit depth changes, the extinction efficiency is expected to change in a wavelength- and composition-dependent way. Observing a change in the wavelength-dependent transit depth would offer an unprecedented opportunity to determine the composition of the disintegrating rocky body KIC 12557548 b. We detected four out-of-transit u' band events consistent with stellar flares.